Method for making large diameter optical waveguide having Bragg grating and being configured for reducing the bulk modulus of compressibility thereof

ABSTRACT

The present invention provides a method for making a multicore large diameter optical waveguide having a cross-section of at least about 0.3 millimeters, two or more inner cores, a cladding surrounding the two or more inner cores, and one or more side holes for reducing the bulk modulus of compressibility and maintaining the anti-buckling strength of the large diameter optical waveguide. The method features the steps of: assembling a preform for drawing a multicore large diameter optical waveguide having a cross-section of at least about 0.3 millimeters, by providing an outer tube having a cross-section of at least about 0.3 millimeters and arranging two or more preform elements in relation to the outer tube; heating the preform; and drawing the large diameter optical waveguide from the heated preform. In one embodiment, the method also includes the step of arranging at least one inner tube inside the outer tube.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit to provisional patent application Ser.No. 60/387,174, filed Jun. 7, 2002.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention generally relates to a method for manufacturing anoptical component; and more particularly to a method for manufacturing alarge diameter cane waveguide having a cross-section of at least about0.3 millimeters, two or more inner cores, a cladding surrounding the twoor more inner cores and side holes for reducing the bulk modulus ofcompressibility and maintaining the anti-buckling strength of the largediameter optical waveguide.

2. Description of Related Art

There are known methods for drawing an optical fiber, as well as a largediameter cane waveguide having a cross-section of at least about 0.3millimeters, two or more inner cores, a cladding surrounding the two ormore inner cores. The present invention relates to and satisfies a needin the industry for drawing such a large diameter cane waveguide with astructural configuration for reducing the bulk modulus ofcompressibility and maintaining the anti-buckling strength of the largediameter optical waveguide.

SUMMARY OF THE INVENTION

Cane elements are ideally drawn from an assembled glass preform composedof tubes, filler rods, and optical preform elements. Standard opticalfiber preform vapor deposition processes such as MCVD, OVD or VAD can beused to fabricate preform elements. Preform elements may be pre-drawn topredetermined diameters to yield proper core size when the assembly isfinally drawn. Filler rods are used to minimize distortion of circularoptical cores during the draw process.

In its broadest sense, the present invention provides a method formaking a multicore large diameter waveguide having a cross-section of atleast about 0.3 millimeters, two or more inner cores, a claddingsurrounding the two or more inner cores, and a structural configurationfor reducing the bulk modulus of compressibility and maintaining theanti-buckling strength of the large diameter optical waveguide. Themethod features the steps of:

assembling a preform for drawing the multicore large diameter waveguideby providing an outer tube having a cross-section of at least about 0.3millimeters, and arranging two or more preform elements in relation tothe outer tube;

heating the preform; and

drawing the large diameter waveguide from the preform.

In one embodiment, the method also includes the step of arranging atleast one inner tube inside the outer tube.

In this embodiment, the at least one inner tube may include three innertubes symmetrically arranged inside the outer tube. The two or morepreform elements may be arranged between the outer tube and the threeinner tubes. The method also includes arranging one or more filler rodsin one or more voids between the outer tube and the three inner tubes,as well as arranging one or more filler rods in a void between threeinner tubes.

In another embodiment, the method also comprises the steps of notchingthe at least one inner tube with two or more notches; and providing arespective preform element in each notch.

In still another embodiment, the method may also include the step ofcircumferentially arranging one or more filler rods between the at leastone inner tube and the outer tube in relation to the two or morepreforms. The two or more preforms may be diametrically opposed, orarranged at 0, 90, 180 and 270 degrees.

In still another embodiment, the method also includes the steps ofnotching the outer tube with two or more notches; and providing arespective preform element in each notch. The notches may bediametrically opposed. This method also includes the steps of arrangingone or more filler rods in each notch.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWING

The drawing, not drawn to scale, include the following Figures:

FIG. 1 shows a preform assembly according to a method for making a largediameter optical waveguide according to the present invention.

FIG. 2 shows a nested tube preform assembly according to a method formaking a large diameter optical waveguide according to the presentinvention.

FIG. 3 shows a nested tube with filler rods according to a method formaking a large diameter optical waveguide according to the presentinvention.

FIG. 4 shows a preform assembly according to a method for making a largediameter optical waveguide according to the present invention.

FIG. 5 includes FIGS. 5( a), 5(b) and 5(c); FIG. 5( a) is a diagram ofan embodiment of a large diameter optical waveguide having multipleholes with a clover leaf design and multiple cores with Bragg gratingwritten therein; FIG. 5( b) is a diagram of an embodiment of a largediameter optical waveguide having multiple holes with a honeycomb-likedesign and multiple cores with Bragg grating written therein; and FIG.5( c) is a diagram of an embodiment of a large diameter opticalwaveguide having a tubular hole and diametrically opposed multiple coreswith Bragg grating written therein.

FIG. 6 is a diagram of a large diameter optical waveguide having a Bragggrating written therein.

BEST MODE FOR CARRYING OUT THE INVENTION FIGS. 1–4: Methods ofManufacture

FIG. 1 shows an example of a tri-core preform assembly design for use ina method for making a multicore large diameter optical waveguide 230(FIG. 5( a) having a cross-section of at least about 0.3 millimeters,two or more inner cores 234, a cladding 231 surrounding the two or moreinner cores 234, and a structural configuration 232 for reducing thebulk modulus of compressibility and maintaining the anti-bucklingstrength of the large diameter optical waveguide. The method includesthe steps of:

assembling a preform generally indicated as 300 for drawing themulticore large diameter optical waveguide 230 in FIG. 5( a) byproviding an outer tube 302 having a cross-section of at least about 0.3millimeters, and arranging two or more preform elements 306 in relationto the outer tube 302;

heating the preform assembly 300; and

drawing the large diameter optical waveguide 231 from the preform 300.

In the preform assembly 300, three inner tubes are symmetricallyarranged inside the outer tube. The two or more preform elements 306 arearranged between the outer and inner tubes 302. Filler rods 304 arearranged in voids between the inner and outer tubes 302 in relation tothe two or more preforms 306. As shown, one filler rod is arranged in avoid between three inner tubes 302. Techniques for heating the preformand drawing a large diameter optical waveguide from a heated preform areknown in the art, and the scope of the invention is not intended to belimited to any particular kind or type thereof.

FIG. 2 shows an example of a ‘nested tube’ preform assembly designgenerally indicated as 310 having nested tubes 312 a, 312 b for use in amethod for making a multicore large diameter optical waveguide. The tube312 b defines the hole 315. One or more of the tubes 312 b is notched313 a, 313 b to accept one or more preform elements 316. The notches 313a, 313 b are cut to produce a minimal gap around the preform element 316in order to reduce core distortion during the draw process. Coredistortion may be further reduced with a core element that is shaped tofit the notch 313 a, 313 b, for example, grinding the preform element316. The preform assembly 310 is heated and drawn to provide the largediameter optical waveguide 250 in FIG. 5( c).

FIG. 3 shows another example of a multi-core preform assembly designgenerally indicated as 320 that is also heated and drawn to provide thelarge diameter optical waveguide 250 in FIG. 5( c). This preformassembly 320 consists of nested tubes 322 a, 322 b with preform elements326 and filler rods 324 located between them. The tube 322 b defines thehole 325. The two or more preforms 326 may be diametrically opposed, orarranged at 0, 90, 180 and 270 degrees, as shown. The scope of theinvention is not intended to be limited to how the preforms are arrangedin the assembly. In this design, some or all of the filler rods 324 maybe substituted with preform elements 326.

FIG. 4 shows yet another preform assembly design generally indicated as330 that consists of a notched tube 332 filled with preform elements 336and filler rods 334. The preform assembly 330 may also be heated anddrawn to provide the large diameter optical waveguide 250 in FIG. 5( c).The notched tube 332 defines the hole 335. The tube 322 b defines thehole 325. The two or more preforms 336 may be diametrically opposed, asshown, or arranged at 0, 90, 180 and 270 degrees. The scope of theinvention is not intended to be limited to how the preforms are arrangedin the assembly. The scope of the invention is intended to includeforming the filler rods 334 as a rectangular cross section shaped glassto minimize core distortion.

FIGS. 5, 5(a), (b) and (c)

FIG. 5 shows three multiple off-center core designs, two of which arediscussed above.

FIG. 5( a) shows the large diameter optical waveguide generallyindicated as 230 including a cladding 231 having multiple holes 232 witha honeycomb design and multiple cores 234 with Bragg grating (not shown)written therein. The multiple holes 232 include three symmetricallyarranged holes 232. The multiple cores 234 include three cores 234symmetrically arranged about the holes 232. The large diameter opticalwaveguide generally indicated as 230 is the result of using the preform300 (FIG. 1) in relation to the aforedescribed method.

FIG. 5( b) shows a large diameter optical waveguide generally indicatedas 240 including a cladding 241 having multiple holes 242 and multiplecores 244 with Bragg grating (not shown) written therein. The multipleholes 242 include four symmetrically arranged holes 242. The multiplecores 244 include three cores 244 symmetrically arranged about the holes242.

FIG. 5( c) shows a large diameter optical waveguide generally indicatedas 250 including a cladding 251 having multiple holes 252 and multiplecores 254 with Bragg grating (not shown) written therein. The multipleholes 252 include four symmetrically arranged holes 252. The multiplecores 254 include three cores 254 symmetrically arranged about the holes252. The large diameter optical waveguide generally indicated as 250 isthe result of using the preform 320 (FIG. 3) in relation to theaforedescribed method.

Similar to that discussed above, the large diameter optical waveguide orcane designs in FIG. 5 allow independent Bragg gratings to be writteninto each core and then be coupled to individual pigtail fibersproviding for a large range of wavelength tuning with a single actuatedcane element. The holes in the cane reduce the effective element modulusenabling a wide range of grating wavelength tuning with reducedcompressive force and buckling.

The scope of the invention is not intended to be limited to the specificnumber of holes 232, 242, 252 or cores 234, 244, 254, or the arrangementof the same in relation to one another, for the designs shown in FIGS.5( a), (b) and (c). Embodiments are envisioned having more or less holes232, 242, 252 or cores 234, 244, 254, as well as different symmetricaland non-symmetrical arrangements of holes 232, 242, 252 and cores 234,244, 254 in relation to one another, than that shown in FIGS. 5( a), (b)and (c).

FIG. 6: The Large Diameter Optical Waveguide

FIG. 6 shows a large diameter optical waveguide generally indicated as40 (also known as a “cane”), which is an example of the structuresdiscussed above. The large diameter optical waveguide 40 has an innercore 42 and an outer cladding 44 surrounding the inner core 42, opposingends 41 a, 41 b, and a diameter D of at least about 0.3 millimeters,similar to that disclosed in the aforementioned co-pending U.S. patentapplication Ser. No. 09/455,868 (CC-0230), which is hereby incorporatedby reference. The inner core 42 has a Bragg grating 22 a written thereinfor tuning by applying a compressive force indicated by arrows 48 on theopposite ends 41 a, 41 b of the optical waveguide 40, or for sensing anexternal parameter like pressure applied thereon.

Cane waveguides have proven to be useful elements for creating highlyreliable tunable grating based elements, and appear to be suitable for avariety of other applications.

One of the issues associated with the tuning of cane waveguides is theforce required to tune a given cane element (typically formed in a“dogbone” element). Reducing the cane diameter can reduce the forcerequired to tune a grating a given amount; however, the element willbuckle at a lower compression strain, ultimately producing a lowertuning range.

The large diameter optical waveguide 40 comprises silica glass (SiO₂)based material having the appropriate dopants, as is known, to allowlight indicated by arrow 45 to propagate in either direction along theinner core 42 and/or within the large diameter optical waveguide 40. Theinner core 42 has an outer dimension d_(c) and the large diameteroptical waveguide 40 has an outer dimension D. Other materials for thelarge diameter optical waveguide 40 may be used if desired. For example,the large diameter optical waveguide 40 may be made of any glass, e.g.,silica, phosphate glass, or other glasses; or solely plastic.

The outer dimension D of the outer cladding 44 is at least about 0.3millimeters; and the outer dimension d_(c) of the inner core 42 is suchthat it propagates only a few spatial modes (e.g., less than about 6).For example for single spatial mode propagation, the inner core 42 has asubstantially circular transverse cross-sectional shape with a diameterd_(c) less than about 12.5 microns, depending on the wavelength oflight. The invention will also work with larger or non-circular coresthat propagate a few (less than about 6) spatial modes, in one or moretransverse directions. The outer diameter D of the outer cladding 44 andthe length L have values that will resist buckling when the largediameter optical waveguide 40 is placed in axial compression asindicated by the arrows 48.

The large diameter optical waveguide 40 may be ground or etched toprovide tapered (or beveled or angled) outer corners or edges 50 toprovide a seat for the large diameter optical waveguide 40 to mate withanother part (not shown herein) and/or to adjust the force angles on thelarge diameter optical waveguide 40, or for other reasons. The angle ofthe beveled corners 50 is set to achieve the desired function. Further,the large diameter optical waveguide 40 may be etched or ground toprovide nubs 52 for an attachment of a pigtail assembly 54 (not shownherein) to the large diameter optical waveguide 40. Further, the size ofthe large diameter optical waveguide 40 has inherent mechanical rigiditythat improves packaging options and reduces bend losses.

In the large diameter optical waveguide 40, the Bragg grating 22 a isimpressed (or embedded or imprinted) therein. A Bragg grating 22 a, asis known, is a periodic or aperiodic variation in the effectiverefractive index and/or effective optical absorption coefficient of anoptical waveguide, such as that described in U.S. Pat. Nos. 4,725,110and 4,807,950, entitled “Method for Impressing Gratings Within FiberOptics”, to Glenn et al.; and U.S. Pat. No. 5,388,173, entitled “Methodand Apparatus for Forming Aperiodic Gratings in Optical Fibers”, toGlenn, which are hereby incorporated by reference to the extentnecessary to understand the present invention. The aperiodic variationof the gratings described herein may include a chirped grating. See alsoU.S. Pat. Nos. 5,042,897 and 5,061,032, both issued to Meltz et al., andhereby incorporated by reference in their entirety. As shown, thegrating 22 a is written in the inner core 42; however, the scope of theinvention is intended to include writing the grating in the outercladding 44, as well as a combination of the inner core 42 and the outercladding 44. Any type of wavelength-tunable grating or reflectiveelement embedded, etched, imprinted, or otherwise formed in the largediameter optical waveguide 40 may be used. The large diameter opticalwaveguide 40 may be photosensitive if the grating 22 a is to be writteninto the large diameter optical waveguide 40. As used herein, the term“grating” means any of such reflective elements. Further, the reflectiveelement (or grating) 22 a may be used in reflection and/or transmissionof light. The incoming light 57 incident on the grating 22 a reflects aportion thereof as indicated by a line 58, and passes the remainingincident light 57 (within a predetermined wavelength range), asindicated by a line 60 (as is known).

The grating 22 a has a grating length Lg, which is determined based onthe application, and may be any desired length. A typical grating 22 ahas a grating length Lg in the range of about 3–40 millimeters. Othersizes or ranges may be used if desired. The length Lg of the grating 22a may be shorter than or substantially the same length as the length Lof the large diameter optical waveguide 40. Also, the inner core 42 neednot be located in the center of the large diameter optical waveguide 40but may be located anywhere in the large diameter optical waveguide 40.

Accordingly, an outer diameter D of greater than about 400 microns (0.4millimeters) provides acceptable results (without buckling) for awaveguide length L of 5 millimeters, over a grating wavelength tuningrange of about 10 nm. For a given outer diameter D as the length Lincreases, the wavelength tuning range (without buckling) decreases.Other diameters D for the large diameter optical waveguide 40 may beused depending on the overall length L of the large diameter opticalwaveguide 40 and the desired amount of compression length change ΔL orwavelength shift Δλ.

The large diameter optical waveguide 40 may be made using fiber drawingtechniques that provide the resultant desired dimensions for the coreand the outer diameter discussed hereinbefore. As such, the externalsurface of the large diameter optical waveguide 40 will likely beoptically flat, thereby allowing Bragg gratings to be written throughthe cladding similar to that which is done for conventional opticalfiber. Because the large diameter optical waveguide 40 has a large outerdiameter compared to that of a standard optical fiber (e.g., 125microns), the large diameter optical waveguide 40 may not need to becoated with a buffer and then stripped to write the gratings, therebyrequiring less steps than that needed for conventional optical fibergratings. Also, the large outer diameter D of the large diameter opticalwaveguide 40, allows the waveguide to be ground, etched or machinedwhile retaining the mechanical strength of the large diameter opticalwaveguide 40. The large diameter optical waveguide 40 is easilymanufacturable and easy to handle, and may be made in long lengths (onthe order of many inches, feet, or meters) then cut to size as neededfor the desired application.

Also, the large diameter optical waveguide 40 does not exhibitmechanical degradation from surface ablation common with optical fibersunder high laser fluency (or power or intensity) during grating exposure(or writing). In particular, the thickness of the cladding between thecladding outer diameter and the core outer diameter causes a reducedpower level at the air-to-glass interface for a focused writing beam.

The large diameter optical waveguide also reduces coupling between thecore and cladding modes due to the increased end cross-sectional areabetween the core and cladding of the waveguide. Thus, the gratings 22 awritten in the inner core 42 of the large diameter optical waveguide 40exhibit less optical transmission loss and exhibits a cleaner opticalprofile than a conventional fiber grating because the large claddingregion dissipates coupled cladding modes, thereby reducing the couplingof the inner core 42 to the outer cladding 44 modes. In general, thegreater the difference in the cross-sectional area between the innercore 42 and the outer cladding 44 the smaller the mode field overlap andthe lower the coupling to the cladding modes. The thickness of the outercladding 44 between the cladding outer diameter and the core outerdiameter may be set to optimize this effect. Other diameters of theinner core 42 and the large diameter optical waveguide 40 may be used ifdesired such that the cladding modes are reduced to the desired levels.

The large diameter optical waveguide 40 may have end cross-sectionalshapes other than circular, such as square, rectangular, elliptical,clam-shell, octagonal, multi-sided, or any other desired shapes,discussed more hereinafter. Also, the waveguide may resemble a short“block” type or a longer “cane” type geometry, depending on the lengthof the waveguide and outer dimension of the waveguide.

THE SCOPE OF THE INVENTION

It should be understood that, unless stated otherwise herein, any of thefeatures, characteristics, alternatives or modifications describedregarding a particular embodiment herein may also be applied, used, orincorporated with any other embodiment described herein.

For example, although the invention is described in relation to longperiod gratings, the inventors envision other embodiments using blazedgratings, periodic or aperiodic gratings, or chirped gratings.

Although the invention has been described and illustrated with respectto exemplary embodiments thereof, the foregoing and various otheradditions and omissions may be made therein without departing from thespirit and scope of the present invention.

1. A method for making a multicore optical waveguide having across-section of at least about 0.3 millimeters, two or more innercores, and a cladding surrounding the two or more inner cores,comprising the steps of: assembling a preform for drawing the multicoreoptical waveguide by providing an outer tube having a cross-section ofat least about 0.3 millimeters, arranging two or more preform elementsin relation to the outer tube, and arranging a hollow inner tube insidethe outer tube, wherein the two or more preform elements supply the twoor more inner cores; notching the inner tube with two or more notchesproviding a respective one of the preform elements in each notch;heating the preform; and drawing the optical waveguide from the preform,thereby defining the cross section that is greater than about 0.3millimeters in all directions upon completion of the drawing, andthereby leaving in an area of the optical waveguide that has been drawna longitudinal aperture extending through the optical waveguide, whereinthe longitudinal aperture is formed by the hollow inner tube.
 2. Amethod according to claim 1, wherein the two or more preform elementsare arranged between the outer tube and the at least one inner tube. 3.A method according to claim 1, wherein the inner and outer tubes areconcentric.